Mechanistically, the actions of LH/CG can be regulated at the level of the LH/CG receptor (LHR) by reducing the number of receptors (down- regulation) or by the functional uncoupling of a constant number of receptors from its effector system (uncoupling). We have previously shown that the hCG-induced down-regulation of the LHR is mostly due to the increased receptor degradation that ensues following endocytosis of the bound hormone. We have also shown that phosphorylation of the LHR is necessary but not sufficient for the hCG-induced uncoupling. The studies proposed herein seek to further our understanding of the molecular basis of uncoupling and down-regulation of the LHR. The specific aims are as follows: (1) identify the kinases that phosphorylate the LHR and the residues that become phosphorylated; (2) examine the involvement of palmitoylation of the LHR on hCG-induced uncoupling; (3) determine the importance of LHR activation in LHR- mediated endocytosis; and (4) examine the molecular basis for the increased down-regulation of the LHR detected in Leydig tumor cells when compared to non-target cells expressing the recombinant LHR. While it is clear that changes in the levels of circulating hormones are effective means of modulating target cell responsiveness, emerging information about the regulation of hormone action at the level of their receptors argues that this level of regulation is equally effective and important. An efficient and highly specific way by which target cells regulate their responsiveness to continuous stimulation is by changing the functional properties or the numbers of hormone receptors. Our studies should thus provide valuable insights about the regulation of the function of the LHR, the glycoprotein hormone receptors and the G protein-coupled receptors in general.